This invention relates to air sampling systems and more particularly to methods and apparatus for removing N-nitroso compounds and amines from air samples and for providing quantitative analyses of the concentration of N-nitroso compounds in the air sample.
Devices and methods are known for sampling air to identify types and levels of organic compounds present in the environment. These compounds may arise from a variety of sources, such as the manufacture and use of chemicals, combustion of fuels, and others. Adsorbent materials are often employed in such devices to filter polluted air or trap measurable quantities of pollutants for analysis. While quantitative analyses of air are generally possible with such materials, the achievement of high sensitivity and consistent high accuracy has been difficult to obtain, particularly for the detection of complex organic compounds, such as nitrosamines, in low concentrations. One problem with use of adsorbent materials is that some adsorbents, such as activated charcoal, typically vary substantially in composition, and their adsorption characteristics, among manufacturers, and between batches from the same manufacturer. Also, many of the conventional adsorbent materials do not effectively trap certain low molecular weight compounds of interest. Even in cases where the compounds of interest are trapped, heat is commonly used to desorb the compounds from the adsorbent for analysis. This heat frequently results in the formation of additional amounts of the compounds whose detection is sought, or alteration of the trapped compounds. The use of certain "gas chromatographic" materials as adsorbents helps avoid variations in composition, but not the heat desorption problem. Moreover, the latter materials typically involve "breakthrough" problems, i.e. during trapping, certain of the compounds of interest gradually migrate through the gas chromatographic material in the direction of airflow and are lost from the collection device, leading to errors in the quantitative analysis. Certain other adsorbents, such as silica gel, may have at least one of the above-mentioned drawbacks. In addition, these other adsorbents exhibit an affinity for moisture which degrades their trapping efficiency and renders them unsuitable for analysis of moist air.
Among the substances whose detection in air and/or removal therefrom are of particular interest in the present invention, are N-nitroso compounds, each of which have the general formula: ##STR1## where R.sub.1 and R.sub.2 are the same or different organic radicals including those radicals which together with the non-nitroso N of the depicted N-NO molecular bond constitute a nitrogen heterocyclic radial.
This group of compounds includes several nitrosamines which are known to be such potent carcinogens that dosages of a few parts per million have been observed to produce cancers in animals. The high carcinogenity of these compounds when inhaled or ingested has caused much concern regarding their occurrence in certain materials and in the environment.
In the continuing search for the presence of nitrosamines and their possible role in causing cancer in humans, considerable interest has developed in monitoring air, particularly in locations near factories and cities where volatile nitrosamines have been found and non-volatile nitrosamines may also occur under certain conditions. However, accurate collection and analysis of samples from atmospheric environments is difficult in comparison to foods and chemicals, samples of which have readily been obtained for measurement of the content of nitrosamines. It is particularly difficult to achieve sufficiently rapid collection and analysis to adequately monitor air whose quality is susceptible to rapid change. Thus, it is important in such applications that the collection apparatus and techniques permit accurate and rapid analysis of the compounds extracted from the air. In addition, the technique and apparatus used to test air for the presence of N-nitroso compounds, whether or not part of a portable system, must permit trapping and retention of substantially all of the compounds in the air sampled.
Moreover, it is also known that nitrogen oxides (particularly NO.sub.2, N.sub.2 O.sub.3, and N.sub.2 O.sub.4) can readily N-nitrosate precursor amines in gas phase reaction to form N-nitroso compounds. It is therefore possible that "detected" N-nitroso compounds may have been artifactually formed by N-nitrosation during the collection and analysis process. Consequently, an accurate air sampling and N-nitroso compound detection system must be effective in atmospheres in which precursor compounds are present in large concentrations.
According to one prior technique of detecting N-nitroso compounds in air, the air sample is bubbled through a dilute alkali impinger trap containing a 1 N solution of KOH (potassium hydroxide). The contents of the trap are then extracted using a solvent, dried and then concentrated. This extract is then analyzed for the presence of N-nitroso compounds. One major disadvantage of this method is that collection efficiency is low for certain N-nitroso compounds of interest such as N-nitrosodipropylamine, over 80 percent of which may pass through the bubble trap. Also, this method requires drying and concentration of the extract as by evaporation, which necessitates precise measurements and controls, is time consuming, and may introduce errors in the analysis.
According to another method which has been used to minitor air for N-nitroso compounds, air is drawn through cartridges containing a porous polymer of 2, 6-diphenyl-p-phenylene oxide, such as Tenax GC available from Applied Science Laboratories, Inc., State College, Pennsylvania. This material traps the nitrosamines by adsorption. The trapped materials are desorbed using heat, then the desorbed materials are analyzed, for example, by capillary gas-liquid chromatography followed by low-resolution mass spectrometry. However, Tenax GC is relatively expensive. More importantly, the desorption by heat step after collection results in inaccuracies since the thermal desorption itself may produce nitrosamines from precursors, such as amines and compounds containing nitrogen and/or oxygen present in the cartridge, or may cause decomposition of some of the trapped N-nitroso compounds. Another deficiency of Tenax GC material is that it may not retain all of the N-nitroso compounds in the air pumped through the cartridge. The collection efficiency has been found in tests to be different for different N-nitroso compounds and after a certain volume of air has been drawn through the cartridge, further monitoring results in the "breakthrough" and loss of certain nitrosamines from the cartridge. The breakthrough volume of air is different for different N-nitroso compounds, being generally lowest for the smaller nitrosamines (such as N-nitrosodimethylamine) which are of maximum interest. These test techniques, as well as additional techniques using activated charcoal, and cryogenic traps, have been ineffective for nitrosamines with respect to artifact formation and collection efficiency.
Accordingly, it is a general object of this invention to provide improved methods and apparatus for removing predetermined organic compounds from air.
It is a more particular object of the invention to provide an improved method and apparatus for removing and determining the concentration of N-nitroso compounds in a sample of air.
It is another particular object of the invention to provide an improved method and apparatus for determining the concentration of N-nitroso compounds in an air sample without artifact formation of nitrosamines from precursors in the sample.
It is yet another object of the invention to provide an improved method for determining the content of predetermined organic compounds in a sample of air wherein the compounds may be trapped and then collected in a volume of solution sufficiently small so as not to require concentration of the solution during the determination.
It is also an object of the invention to provide apparatus for collecting organic compounds from air which is compact, inexpensive, portable, and simple to use.